The present invention concerns the field of electric motors including at least three stator coils and, more particularly, a device for controlling such a motor including detecting means for detecting the instantaneous angular position of a rotor.
A  less than  less than brushless motor greater than  greater than  or a motor with no commutator greater than  greater than  is defined in the present description as a direct current electric motor which includes a mobile part (or  less than  less than rotor greater than  greater than ) provided with a permanent magnet, and a fixed part (or  less than  less than stator greater than  greater than ) provided with at least three fixed bipolar coils. In the event that the motor includes three stator coils, they are arranged so as to be staggered at 120xc2x0 to each other. A motor of this type has the advantage of being able to be sterilized, for example in an autoclave, unlike motors with brushes, whose brushes decompose during sterilization. The question of sterilization thus becomes a primary concern, in particular in application fields requiring optimum sanitary hygiene, such as the field of medical instruments.
FIG. 1 shows schematically a brushless motor 1 provided with three stator coils 2 to 4, the motor being controlled by a conventional voltage generator 5. For this purpose, each of coils 2 to 4 includes a connection terminal. References 2a, 3a and 4a respectively designate the connection terminals of coils 2, 3 and 4. Generator 5, includes three connection terminals 5a, 5b and 5c connected respectively to terminals 2a, 3a and 4a, and it is arranged to be able to provide, via terminals 5a to 5c, three respective electric voltages Ua to Uc to the three respective coils 2 to 4, which achieves the control of motor 1.
FIG. 2 shows three timing diagrams 10 to 12 of voltages Ua to Uc when the motor of FIG. 1 is being controlled. It will be noted that the set of these three voltages constitutes a three phase system formed of square periodic signals, these signals having the same amplitude designated A and the same period designated T, and being phase shifted by T/3 with respect to each other. In FIG. 2, the reference t0 designates any initial instant.
When voltages Ua to Uc of FIG. 2 are applied to the respective coils 2 to 4 of FIG. 1, the coils can be polarized sequentially in accordance with six different states. The first state corresponds to the interval of time comprised between instant t0 and t0+T/6, during which voltages Ua, Ub and Uc respectively have the value A, 0 and A. The second state corresponds to the interval of time comprised between instants t0+T/6 and t0+2T/6, and so on. As a result of this polarization, a rotating field able to cause the rotor to rotate is generated, the permanent magnet of the rotor being arranged in close proximity to coils 2 to 4. By way of example, the rotational speed of the rotor can vary between 0 and 40,000 tr/min, and the frequency F0 corresponding to period T can be comprised between 1 and 667 Hz.
One problem encountered with a control of this type is that it is necessary to detect the instantaneous angular position of the rotor. In order to cause the rotor to rotate to a desired angular position, the rotor angular position must be detected at the instant when control voltages are applied to the stator coils, so that the values of the voltages applied can create, in the air gap, a magnetic field able to cause the rotor to rotate from the angular position thus detected to the desired angular position.
A first conventional solution to the problem of detecting the rotor angular position consists in fitting such a motor with a coding device which is linked to the rotor, and which controls the switching of the electric voltages applied to the stator coils. For example, contactless electronic sensors are commonly used to detect the rotor angular position, said coding device including a magnet which rotates with the rotor, and several cells or Hall effect sensors situated in the field of the magnet, and fixed to the stator so as to switch when there are magnetic field reversals. It will be recalled that a Hall effect cell is arranged to be able to detect the variations in a neighbouring magnetic field.
A solution of this type has various drawbacks. In particular, Hall effect cells are relatively expensive and the mounting thereof in proximity to the motor increases the space requirement of the latter. Furthermore, it is necessary to use, in addition to the three power supply wires of the motor, two wires for supplying power to the Hall effect cells and three wires for collecting the data provided by thereby. It goes without saying that the arrangement of an instrument with eight wires goes against constraints as to handling ability, sterilization, weight, robustness and cost, these constraints being common in industry, in particular within the field of medical instruments.
A second conventional solution to the problem of detecting the rotor angular position consists in measuring the back-electromotive force which is proportional to the rotational speed of the rotor and which, consequently, can provide data relating to its movement and thus to the speed of the rotor.
One drawback of such a solution lies in the fact that it does not allow the rotor angular position to be detected directly.
Another drawback of this solution lies in the fact that this force decreases with the rotational speed of the rotor, which makes it difficult to measure.
Another drawback of this solution lies in the fact that detection of the instantaneous position of the rotor disturbs the normal operation of the motor. The back-electromotive force measurement can not be performed simultaneously with the supply of the motor control voltages. Thus, the working of the motor is interrupted at each back-electromotive force measurement.
It has thus been observed that all the solutions proposed in the state of the art to answer the aforementioned problem were not satisfactory for, on the one hand, detecting the instantaneous angular position of the rotor and, on the other hand, answering constraints or concerns belonging to specific application fields, for example for controlling a brushless motor in a dental instrument. Also by way of example, within the scope of an application to robotics, the conventional solutions do not allow the rotation of the rotor from a predetermined angle to be controlled with sufficient precision and with a small number of lead wires.
One object of the present invention is to provide a control device able to be connected to a brushless motor, this device overcoming the aforementioned drawbacks and, in particular, being able to detect the instantaneous angular position of the motor rotor.
Another object of the present invention is to provide a control device able to be connected with a minimum of connecting wires, so as to answer the constraints as to handling ability, weight and cost, such constraints being common in industry, and in particular in the field of medical instruments.
Another object of the present invention is to provide a control device able to detect the instantaneous angular position of the rotor independently of the rotational speed of the rotor, even if the rotor has stopped.
Another object of the present invention is to provide a control device able to detect the instantaneous angular position of the rotor without disturbing the normal operation of the motor.
Another object of the present invention is to provide a control device answering the constraints as to sterilization and robustness, in particular for an application in the field of medical instruments.
These objects, in addition to others, are achieved by the control device according to claim 1.
The device according to the invention includes detecting means which have the advantage of providing at least two measurement signals at respective frequencies which together represent the rotor angular position, and that the supply of these signals is independent of the normal operation of the motor, in particular the control of the latter by the power supply means.
Another advantage of these detecting means lies in the fact that they are connected to the power supply wires connecting the motor to the power supply means, without requiring additional connections between the motor and the detecting means to be able to detect the rotor angular position. As a result, the motor fitted with such an acquisition device answers the concerns as to handling ability, robustness and weight, which are common in industry, in particular in the field of dental instruments.
The device according to the present invention further includes filtering means which have the advantage of filtering the measurement signals across the output terminals of the power supply means, so as to assure that the supply of these signals does not disturb the control of the motor by the power supply means.
The device according to the present invention further includes measurement means which have the advantage of providing the measurement signals whose frequencies are higher than the frequency of the control signals, and whose amplitudes are lower than said control signals, so that the measurement signals do not interfere with the control signals when the motor is controlled, so as to assure that the supply of the control signals does not disturb the control of the motor via the power supply means.
Another advantage of the detecting means lies in the fact that they include inexpensive, non complex and compact electronic components, which answers the usual industrial concerns as to price, rationalization and compactness.
Another advantage of the control device according to the present invention lies in the fact that they allow, in particular when the rotor has stopped, the instantaneous angular position of the rotor to be detected, without inducing any effect on the inductive distribution of the rotating field present in the motor air gap. Those skilled in the art will note that the measurement signals at their respective frequencies can be provided for any value of the rotational speed of the rotor, and in particular when the rotor has stopped, since the detection of the angular position is derived from the measurement of the measurement frequencies which are provided independently of the control of the motor by the power supply means.
Another advantage of the control device according to the present invention lies in the fact that it allows the instantaneous angular position of the rotor to be detected with a precision of the order of a degree, which answers a demand for precision, which is common in industry, in particular in the field of robotics and dental implantology.
The device according to the present invention further includes processing means which have the advantage of comparing the calculated angular position with a comparison value, which allows the evolution of the angular position to be monitored over time.
Another advantage of these processing means lies in the fact that they allow the number of revolutions made by the rotor to be calculated, was well as the rotor speed and acceleration, which allows the evolution of these parameters to be monitored over time.